1. Field of the Invention
The present invention relates to an image reading apparatus for illuminating a film bearing an image and reading said image from the light transmitted by the thus illuminated film.
2. Related Background Art
There has already been proposed an apparatus for high-density recording of the information generated in a large quantities, such as of documents, onto microfilms and for reading the recorded image on said microfilms, when required, for reproducing on a recording sheet.
In such apparatus the microfilm is exposed to light, and the density of the light transmitted by the microfilm is detected by an image sensor such as a CCD to obtain an electrical image signal.
For converting the thus obtained image signal for example into a binary signal representing black and white, there is generally conducted a comparison of said image signal with a predetermined threshold value. However the density of image recorded on the microfilm is variable according to the photographing condition, and the use of a fixed threshold value for binary encoding may result in a very dark or faint output image. It is therefore conceived to vary the threshold value according to the density of the recorded image.
Consequently, it is known to photoelectrically detect the amount of light transmitted by the microfilm, in order to determine the threshold value according to the density of recorded image. However, the size and position of the images recorded on the microfilm are not constant, so that, if the threshold value is determined, for example, by the light transmitted by the entire area of film, the exact image density cannot be detected as the transmitted light through a non-image area affects the determination of the threshold value.
On the other hand, it is also conceivable to determine the threshold value from the peak density of the recorded image. However, if characters and symbols of different sizes are present in an image, an image density corresponding to a large character is detected as the peak value, and small characters may be omitted in the reproduced image when binary encoding is conducted with a threshold value determined according to such peak value.
In addition, the background or base density varies according to the type of the microfilm, and the image contrast varies according to such base density even if the image density recorded on the film is constant. Consequently, a threshold value, which is determined solely by the image density recorded on the film, may not be able to achieve exact binary encoding of images close to the background or base density.
Also, the output of the image sensor may fluctuate due to time-dependent change or the circumferential condition of the light source and of the image sensor, thus affecting the determination of an appropriate threshold value.
The output image signal is influenced by the amount of light emitted from the light source for illuminating the microfilm. It is therefor conceivable to control the amount of light at a constant value. However, as explained above, the amount of light transmitted by the microfilm is governed by the base density thereof, which is variable according to the type of film and the developing conditions thereof. Consequently, a constant amount of light alone does not ensure satisfactory image reading from different films.
It is therefore conceivable to detect the amount of light transmitted by the film and to regulate said amount of light to be always to an optimum value regardless of the base density. However, such method of control with the transmitted light is unable to achieve appropriate control in the case that film has a local smear or scar.
Also, in such conventional apparatus, the image reading of microfilm has required an operator for confirming that an image frame on the microfilm is projected in a correct position, through inspection of a projection screen, since otherwise a non-image area between image frames on the film may be erroneously read.